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Commonly used plant terms explained!

What is a stem?  Is it the same as a petiole? 
Does your plant grow from a corm, a bulb or a tuber and what is the stalk that supports a single flower?
Is a stem the stalk that supports a leaf and other things that you may have learned incorrectly!

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Regardless of what the folks at your local nursery or discount plant store told you the stalk that holds a leaf in place is not a "stem".   Look up the definition of the stem in a botanical dictionary and you will learn a stem produces roots and nodes.  Those nodes produce the stalk that supports the leaves and that support is technically a part of the leaf.  Take a look.  You won't find a root or a node growing on the stalk which supports a leaf so it cannot be a stem!  
So you've been told your plant grows from a bulb?  Maybe not!  It may grow from a corm, a rhizome or a tuber.  There is a difference and most growers have been given bad information.  Sound confusing? 

Although I doubt this article will change the use of the term "stem" or bulb as it has been adopted by house plant growers and in horticulture, regardless of the species any botanist will tell you the stalk that supports a leaf is correctly known as a petiole, not a stem and the underground starch storage unit of a bulb, corm, and a tuber are distinctly different.
There are also two additional words that are only slightly related but for the sake of those doing a search on the internet we define those as well.  The first is a pedicel and the second is a peduncle. 
Philodendron plowmanii inflorescence, spathe, spadix, floral chamber and peduncle, Photo Copyright Steve Lucas, www.ExoticRainforest.comA pedicel is the support of a single flower.  It can be applied to any flower and the stem is not the stalk that supports either a leaf or a flower.  The support of a flower is correctly known as a pedicel and the term applies whether you are talking about a large flower or a microscopic flower.  Pedicels can even be observed in the tiny near microscopic flowers of an inflorescence.
The term peduncle refers to the stalk that supports an inflorescence.  Aroids produce an inflorescence, or group of tiny flowers as do gingers and other plants.  Despite common believe, the often colorful spathe of an aroid is not a flower but is only a modified leaf.  The flowers are found only on the spadix at the center of the inflorescence.
A stem is the plant's main support, base or central axis and its roots anchor the
plant either to the ground, a tree or to a rock.  A stem may also be a rhizome as well as a tuber (often incorrectly called a corm or a bulb) and grow beneath the soil.  A stem may even spread as a repent rhizome creeping across the soil (repent indicates the rhizome runs roughly horizontally).  A rhizome is just a stem (central support) that runs either along or just beneath the surface of the soil as can be seen with Ginger plants or with an Iris. 
There are significant structural differences between a bulb, a corm and a tuber.
A bulb is an underground storage structure used to store starches and water that is only a condensed stem since it is the central support of the plant.  A bulb normally has a basal plate and true bulbs have the roots growing from the bottom.  Bulbs also have layers of fleshy storage leaves surrounding the bud that will form the next plant.  The modified leaves are arranged in layers that are used for food storage.  The perfect example of a bulb is an onion, just slice one open and you can easily see the modified leaf layers.  The term bulb is used mostly in horticulture but never applies scientifically to an aroid including Colocasia or Amorphophallus species.
A corm is also an underground storage stem to which the above-ground parts may die back in the dormant season but the term is also not applicable to aroids.  A corm often stores starch and when the corm re-grows foliage the new growth Amorphophallus konjac divided leaf, rachis, petiole, Photo Copyright 2008, Steve Lucas, www.ExoticRainforest.comwill come from the top with the roots growing from its base.  The roots of a corm always grow from the bottom, not from the top. 
Bananas grow from corms as do Gladiolas but the term corm is not scientifically appropriate for an aroid according to more than a few top aroid botanists.  There are a select number of species that have a corm-like tuber but the majority of scientists agree the term does not apply in the family Araceae (aroids) as a result of the position of the roots and lack of a basal plate on an aroid tuber.
A tuber is a strongly condensed stem as well as an underground structure which is almost entirely a starch storage organ without a basal plate.  The buds for future growth and the roots all develop at the apex or top of the tuber
The tuber can sometimes form as the tip of a stolon or runner.  Stolons are stem runners or stem shoots that run atop or just beneath the surface of the soil's surface to produce new plants.  According to scientist Christopher Rogers, "In Amorphophallus, Arum and Tymphonium for example, the stem tissue is all encased the the small bud at the top of the tuber.  That bud grows upwards into a leaf or two and outward into roots with the tuber beneath."  A potato is the perfect example of a tuber and all aroids that develop from an underground stem including Alocasia, Colocasia, Xanthosoma, Caladium and Amorphophallus grow only from tubers.  Just look at the position of the roots.
For a more detailed explanation including drawings and charts visit The University of Illinois Extension   If the explanations above are not adequate you will understand the differences once you view the drawings at this link.
Plants such as Amorphophallus species have their stem (tuber) growing underground.  With only a few exceptions, when in the vegetative state that tuber produces a single petiole that supports a single divided leaf.  The leaflets may appear to be individual leaves but they are all a part of the divided leaf and are supported on a rachis growing from the petiole.
If you are visiting this page as a result of the recent blooming of Amorphophallus titanum at the Houston Museum of Natural Science there is a diagram below of the parts of that species.  for more information on how it pollinates visit this page on aroid pollination,
Even though we correctly call the stem of that genus a tuber it is still just a stem that produces a single petiole which supports a single divided leaf.  Botanist Dr. Wilbert Hetterscheid is the world authority in the genus Amorphophallus and explains, "The tuber is indeed the strongly condensed underground stem consisting in most species of only one node being renewed every year with few exceptions existing."

I've attempted to explain this on many plant forums but someone will almost always come back and say the information I
offer is either wrong or irrelevant.  Often they'll grab a definition from the internet without ever reading what the definition is attempting to explain or will change the facts to fit their own preconceived notion.  The use of a wrong name to describe one of a plant's most distinctive parts is virtually the same as calling a cat a dog or calling a boy a girl.  You wouldn't purposely call your child by the wrong name so why do many of us continue to call the petiole something that is not correct?

Even though I've researched this information for several years, just to be sure I am about to explaining this accurately I sent botanist Peter Boyce in Malaysia a quick note to ask
"Is there ever a situation where the support for a leaf is correctly known as a "stem"? Pete was in a hurry to catch a plane to Thailand but responded,  "The quick answer is never."  He then briefly continued while trying to get out the door saying the use of that term as a leaf support is "very poor use of the word stem."   Pete has described and published many tropical plant species in various genera of the family Araceae (aroids) to science and is one of the co-authors of the scientific text The Genera of Araceae published by the Royal Botanic Garden Kew in London. His co-authors are botanists Simon Mayo and Josef Bogner.

I sent the same question to my friend and mentor Dr. Tom Croat and he quickly responded,  
"Steve, it is very simple. The stem has leaves which extend outward. These each have petioles and each petiole has a blade."   Please notice Dr. Croat explained each leaf has a petiole and the petiole is the stalk that supports the leaf blade. The problem appears to be the way individuals interpret the definition of a stem as it is often explained on the internet.

Some internet definitions clearly don't make a distinction between a stem and the petiole since the petiole is a part of the entire leaf structure.  That lack of a clearly understandable definition combined with the common incorrect usage of the world "stem" may have caused most plant collectors to use the term incorrectly. 

Again I asked Dr. Croat to explain about the parts of a leaf,
"A leaf consists of a petiole and a blade. A petiole may also have a sheath and a geniculum. As a further twist the geniculum actually extends briefly onto the blade at times so is structurally a part of both the blade and the petiole even though we consider it to be technically a part of the petiole."  

When you read sites such as Wikipedia which includes statements such as "A stem is one of two main structural axes of a vascular plant" their statement is both true and scientifically correct but part of their further explanation can be confusing.  They include, "The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaves, inflorescence (flowers), cones or other stems etc."  Their article is talking about the entire leaf unit, not just the leaf blade.  The "other stem" is known as a shoot as in the case of a rose plant but the support for the flower itself is neithr a stem nor a petiole.  The second main structural axis mentioned in the article on Wikipedia is the root system.   (Read further for more information on the rose.)

A geniculum is not found on all plant species but is common on plants such as an Anthurium and Monstera species. The geniculum allows the leaf blade to rotate to orient itself to the light just as your knee or elbow allows you to rotate your leg or arm.

A petiole can take on many shapes including to possess a round shape known as being terete or "U", "C" or "D" shaped when viewed as having been cut as a cross section.  The "U" and "C" shaped petiole are also known as being either sulcate or canaliculate  as in to possess a canal,  A petiole may even have very odd shapes such as being quadrangular or hexagonal and may even have patterns including lines which are known as being striate or grooves which are also known as being sulcate since the grooves form tiny canals in the petioles' axis.  A petiole may also be hard or somewhat spongy to the touch.   Botanists often use the shape and features of the petiole to determine the species of the plant.

The stem of a plant may be repent and run across the ground as a rhizome or or it may stand erect.  In many tropical species the stem often climbs a tree.  The stem is always the plant's base and produces buds, nodes and Philodendron pusillum petiole, Photo Copyright 2009, Steve Lucas, www.ExoticRainforest.cominternodes.  The purpose is to collect and store water as well as nutrients which are absorbed by the roots. The water and nutrients are then distributed to the leaves through the plant's xylem via the petioles. The xylem transports water and soluble nutrients collected by the roots and stored in the stem  into the petiole and up to the leaf lamina.  The water is not "pushed" up the petiole but is instead "drawn" to the leaf lamina as a result of evaporation through the leaf blade.
Buds and roots grow from the nodes on the stem and the buds then produce the petioles. In many tropical plant species those roots never touch the soil since many grow suspended on the trunk or limb of a tree!  In climbing species the stem is also found clinging to the host tree and held in place by the clasping roots.

The use of the word "stem" to indicate the support for a leaf by plant growers may have begun as a result of the beautiful rose. When we buy a dozen roses we find the flower appearing to be at the end of a stem. In the case of the Alocasia zebrina stem, Photo Copyright 2009, Steve Lucas, www.ExoticRainforest.comrose flower that final support stalk actually is a pedicel since it is a non-woody shoot which grows from the true stem which is likely climbing some support. 
When we pick up a rose in a florist shop the leaves have almost always been removed so we forget those leaves also had a support stalk and that support was a petiole. The flower is known as a terminal flower supported on a pedicel at the end of a stem shoot.  The pedicel is is very different from the support for any leaf but neither are correctly as "stem".
Lost?  The rose flower is on the end of a stalk (pedicel) which grows at the end of a stalk (stem) but the leaves grow on their own stalk (a petiole).

The stem is not the stalk that supports each leaf but is instead the main support of the plant and is composed of nodes and internodes producing roots, petioles and the sexual parts of the plant (flowers).  Just take a look at the stalk that supports any leaf.  You won't  find a node or internode growing from either a petiole or pedicel stalk. 
I'm sure the term "stem" will stay in common usage as the support of a leaf but at least you know the support of any leaf blade is not a "stem".   The support for a leaf is a petiole.  The support for the plant is a stem.

Dr. Thomas B. Croat, Missouri Botanical garden research greenhouse, Photo Copyright Janice Lucas, www.ExoticRainforest.comThis text was researched and written by Steve Lucas: 

Although as non-technical as possible the information on this page is based largely on the scientific journals of botanist Dr. Thomas B. Croat Ph.D., P.A. Schulze Curator of Botany of the Missouri Botanical Garden in St. Louis, MO.  Dr. Croat (CROWat) has located an incredible number of newly discovered species and described them to science.  When you see the name "Croat" at the end of a plant's scientific name it was described and likely discovered by Dr. Croat.  Tom was honored in the summer of 2008 for having collected over 100,000 specimens in the wild.  Also, my thanks to botanist Peter Boyce in Malaysia as well as botanist Dr. Wilbert Hetterscheid in the Netherlands for their input and continued assistance.


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Philodendron bipinnatifidum stem, Photo Copyright 2008, Steve Lucas,



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